Multiphase reactor design incorporating filtration system for fixed--bed catalyst

ABSTRACT

A unique reactor configuration especially suitable for interphase mass transfer and mixing of multiple phases, i.e. gas(es), liquid(s), and solid(s) where reaction is catalyzed by a solid catalyst comprises a draught tube reactor wherein the solid catalyst particles are maintained in an annular space between the draught tube of the reactor and an annulus-defining wall by means of filter elements positioned downstream and optionally also upstream from the catalyst bed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to a reactor configuration especiallyuseful in heterogenous reactions employing a solid catalyst in a liquidphase, and to an olefin epoxidation process employing such a reactor.

2. Background Art

The majority of propylene oxide produced today is produced by so-called“coproduct” processes in which an easily oxidizable substrate isoxidized to produce hydroperoxides and/or peroxides, which are then usedto “indirectly” oxidize propylene. The reduction products of theoxidized substrate are generated in large quantities and sold ascoproducts. Typical coproducts are styrene and methyl-t-butylether.Since the process necessitates purchase of the oxidizable substrates andsale of coproduct, the price of each of which may vary widely, “directoxidation” processes have been sought wherein market fluctuations do notdictate the overall economy of the process.

While epoxidation of ethylene with oxygen over a supported silvercatalyst has been widely used, an analogous oxidation of propylene isnot viable. Recent research activity directed to “direct oxidation” ofpropylene has concentrated on use of hydrogen peroxide, generatedexternally or in situ, in the presence of titanium silicate zeolitessuch as titanium silicalite as catalysts. The solid crystalline catalystparticles may be treated to contain a noble metal which catalyzeshydrogen peroxide production from hydrogen and oxygen. If such a processcould be commercially practiced, only low cost reactants would be used,and no coproduct produced.

In U.S. Pat. No. 6,376,686, an olefm epoxidation process is describedemploying a solid catalyst in a reactor configuration similar to thatdisclosed in U.S. Pat. No. 5,972,661, herein incorporated by reference.In the latter patent, a “draught tube” reactor is disclosed, asimplified schematic of which is illustrated by FIG. 1. The reactantslurry, including solid catalyst, enters the central draught tube and isdirected along the axis of the draught tube by impeller(s) therein. Aseries of vertically oriented baffles positioned between impellersprevents a swirling flow which might cause solid catalyst segregation.Upon reaching the end of the draught tube, the slurry flows in acountercurrent direction through an annulus between the draught tube andthe reactor wall. Product is continuously removed, separated fromentrained catalyst, and worked up to remove solvent, byproducts, etc.Catalyst must be returned to the reactor. The reactor configuration hasbeen found to exhibit high mass transfer and mixing rates. The catalystis a combination of active ingredient and inert binder. Under intenseagitation, the catalyst can gradually undergo break-up producing finesthat lead to pluggage of filtration equipment. The fines could also passthrough the filtration equipment resulting in catalyst loss from thereactor-filter setup.

It would be desirable to produce a reactor configuration which can takeadvantage of the mass transfer characteristics of a draught tube reactorwhile causing less catalyst attrition.

SUMMARY OF THE INVENTION

The present invention pertains to a draught tube reactor wherein thecatalyst is located in a fixed bed in an annulus between an internaldraft tube and the reactor wall, the catalyst maintained in the annulusby means of filter media preferably located both above and below thecatalyst bed. Because the catalyst is not circulated as a slurry,catalyst attrition is minimized, allowing use of a variety of catalystparticles, including catalyst particle agglomerates.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the use of a prior art draught tube reactor in areaction wherein a slurry of solid catalyst is circulated.

FIG. 2 illustrates schematically a draft tube reactor of the presentinvention wherein a fixed bed of catalyst is employed in the reactorannulus.

FIG. 3 illustrates one embodiment of a draught tube reactor inaccordance with the present invention employing a catalyst basket;

FIGS. 4 a-4 e illustrate construction of one embodiment of a catalystbasket.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates a prior art draught tube reactor 1 which may be usedfor a heterogeneous reaction, and contains slurried solid catalyst.Within the reactor shell 2 is draught tube 3 into which relevant feedstreams are introduced, here a liquid feed line 4 and vapor (gas) feedline 5. Within the draught tube are impellers 6 driven by motor M whichinduce upwards flow through draught tube 3 and thorough mixing of thereactant feed streams and catalyst slurry. At the top of draught tube 3,the reaction mixture slurry flows countercurrently through the annularspace 7 between draught tube 3 and the reactor wall 2. A portion isrecirculated back through the bottom of draught tube 3, while a furtherportion is taken off at outlet 8 and pumped by circulation pump 9through filter 10. Filtered product stream 11 is removed and processedto remove product, unreacted starting materials, recycle solvent, etc.,while a solid catalyst enriched slurry 12 is returned to the reactor.Catalyst fines generated by attrition of solid catalyst in the reactor,pump, and circulation lines accumulate on filter 10, eventually pluggingthe filter.

In the present invention, solid catalyst particles are maintained in anannulus between the central draft tube and the reactor walls. Withreference to FIG. 2, the central draft tube 21 contains impellers 22mounted on shaft 23 driven by motor M. Between draught tube 21 and thereactor wall 24 is an annulus 25 which contains solid catalyst.Surmounting the annulus 25 is a filter plate 26. A second filter plate27 is located at the bottom of the annulus. The filter plates may bemade of sintered metal, ceramic, metal mesh, etc. Reactants enter thereactor through one or more inlets 28, and are intimately mixed in thedraught tube, which optionally contains baffles 29 between theimpellers. Upon reaching the end of the draught tube, the liquidcontents are directed through filter plate 26 down through the catalystbed, and through filter plate 27. A portion of the liquid reenters thedraft tube, while a portion is drawn off at outlet 30 for separation ofproduct and appropriate treatment/recycle of other components.

The reactor of the present invention is particularly useful wherestreams of various reactants, preferably also including gaseousreactants, must be intimately mixed and contacted with solid,heterogeneous catalyst. The reactor is particularly useful for “direct”olefin epoxidation reactions, but its use is not limited thereto. In thedescription which follows, olefin epoxidation will be used to illustratethe use of the reactor.

In the case of propylene epoxidation, for example, the epoxidationcatalyst may be palladium treated titanium silicalite crystals of largesize such that reasonable back pressure can be maintained, or may berelatively large agglomerates of titanium silicalite crystals such asthose disclosed in U.S. Pat. Nos. 5,500,199 and 6,106,803. The reactantfeed streams may comprise propylene, hydrogen, oxygen, liquid phase,i.e. methanol, inert gas such as a volatile hydrocarbon or nitrogen,carbon dioxide, argon, etc. The product stream will ordinarily compriseunreacted gases which can be separated and recycled, solvent which isordinarily recycled, and propylene oxide and “propylene oxideequivalents,” i.e. ring opened and various condensation products. Thepropylene oxide product is separated and purified by methods well knownto those skilled in hydrocarbon processing, preferably by a series offractional distillations. Any attrited catalyst will accumulate in thefirst distillation bottoms unless previously separated by othertechniques such as centrifugation. Recovered catalyst fines may beprocessed to recover noble metal values.

Because the catalyst particles are not exposed to the high shear forcesof impellers or pumps, attrition is very low. Any fines generated byattrition can be allowed to accumulate in distillation bottoms forperiodic removal rather than employing small pore filters subject toblockage. However, due to the small quantity of fines, filtration orremoval by centrifugation or other techniques remain options with thepresent reactor.

The reactor design is preferably such that filter elements may bechanged periodically. The filter elements generally also requirephysical support. A reactor configuration allowing for such features isillustrated in FIG. 3, although numerous methods of supporting thefilter elements will suggest themselves to those skilled in chemicalengineering and reactor design.

In FIG. 3, a generally cylindrical reactor 30 contains a “catalystbasket” 31 as an integral assembly. The catalyst basket is preferablyclose fitting to the interior wall 32 of the reactor. Fluid flow aroundrather than through the catalyst basket is prevented by o-ring seals 34and 35, although other arrangements may also be suitable. For example, areceiving land may be made in the wall of the reactor on which thebasket is mounted. Mounting may be effected, for example, by a ring ofbolts, and a gasket may be placed between the receiving land and thebasket. Although the entire reactor is generally maintained at highpressure, the pressure is substantially equalized over the entireinterior volume, and thus the o-rings, gaskets or like devices need onlybe able to handle the pressure differential dictated by liquid flow. Inmany cases, o-rings, gaskets and the like may be dispensed with. Thebasket surrounds draught tube 21 and impellers 22. In some designs, aninner wall of the basket may constitute the walls of the draught tube.

The catalyst basket 31 may be assembled by any techniques acceptable inchemical engineering and metal fabrication, for example by assembly ofthe parts shown in FIGS. 4 a-4 e. In a preferred embodiment, a top ring36 is machined with lands 38 and 40 to receive an inner tube 37 and anouter tube 39, the inner tube serving as the draught tube of the reactoror a portion thereof, or having dimensions so as to surround a fixedcentral draught tube. A bottom ring 48 with similar lands 42 and 44 isalso prepared. Ordinarily, the inner and outer tubes are welded to thelands to produce the basic catalyst basket. The top ring is shown fromabove in FIG. 4 c which also illustrates the bottom ring from below.

A top cover 49 which spans the annular space between the outer sleeve 39of the catalyst basket and the draught tube 37 is mountable onto the topring. The cover contains through passages 41 (refer to FIG. 4 b), and atop filter element 42 is generally positioned between the top cover andthe top ring 36. If the fluid flow is from the top of the draught tubethrough the top of the annular space of the filter basket, the topfilter element 42 may be quite coarse in pore size, and may take theform of one or more stainless steel mesh layers, for example. In othercases, so long as fluid flow is constantly maintained, the topmostscreen may be dispensed with. However, such an embodiment is notpreferred. FIG. 4 b illustrates the top and bottom covers from above andbelow. FIG. 4 c represents a top view of ring 36 showing portions 47 bwhich allow the top cover 49 to be mounted to the top ring 36 by holeslocated in corresponding structures 47 a. FIG. 4 d illustrates the topand bottom filters, while FIG. 4 e illustrates the relationships betweenthe inner and outer tubes.

At the bottom of the filter basket is a similar bottom cover 45. Betweenthe bottom cover 45 and the bottom ring is secured a bottom filterelement 46. This bottom filter element 46 should be manufactured with apore size so as to retain the solid catalyst, except for a minor amountof catalyst fines. A suitable nominal pore size may be, for example,4-10 μm. It is expected that some catalyst fines with sizes less than 1μm will be able to pass through such a filter element. This isparticularly so of particles of about 0.2 μm or less, which are mostproblematic in plugging catalyst filters. However, the amount of suchfines will be quite small. The bottom filter element 46 may be made offine mesh screen, but is preferably a sintered metal or ceramic filterelement. If a bed of very fine catalyst particles, i.e. <1 μm in size iscontemplated, the bottom filter element will be made of more finelypored material.

A reactor may contain a single catalyst basket or a plurality of basketswith no sealing means therebetween or with conventional seals, forexample o-rings, gaskets, etc. Prevention of liquid flow from betweenthe baskets rather than from end to end may also be prevented by liningthe composite draught tube with a tubular liner, or by other means. Forexample, the reactor may contain a draught tube fixed in position, andcatalyst baskets may be provided with a central opening having aninternal diameter such that the basket may be inserted between thereactor wall and fixed draught tube.

The use of catalyst baskets allows the baskets to be easily replaced ina reactor to supply fresh catalyst, repair filter elements, etc.,without the down time which would be required with other reactordesigns. However, a reactor may be assembled in any way deemedcommercially feasible. For example, when filter elements are sinteredstainless steel, they may be supplied in the form of disks with acentral hole, and welded both to the draught tube on the one hand andthe reactor wall on the other. It is preferable that the design of thefilter elements and/or their mode of mounting into the reactor orcatalyst basket allow for removal of spent catalyst and replacementthereof. In designs where the filter pores are made purposefully largesuch that fines may escape the reactor, provision may also be made forinserting additional catalyst during operation, for example in the formof a slurry of fresh catalyst introduced directly into the annular spaceof the reactor or into a catalyst basket, when used.

The direction of flow through the draught tube is preferably establishedby the impellers, although upward flow of gaseous or liquid reactantscan be used to establish flow in impeller-less configurations. Whenimpellers are used, flow may be upwards or downwards. Reactors may alsobe positioned horizontally. Vertical reactors with upward,impeller-driven flow through the draught tube are preferred.

Reactant, solvent, and other feed streams are generally introduced intothe reactor near the end of the draught tube or within it. The excellentmixing action of the impellers in the draught tube mix the ingredientsvery effectively. Maximum solubility of gases such as hydrogen andoxygen is rapidly achieved for example. By the time the liquid in thedraft tube exits the tube and begins flow through the catalyst bed, thereactants have been thoroughly mixed.

As the reactants flow through the catalyst bed, the catalytic processesoccur, and products, byproducts, unreacted starting reactants, etc. exitthe catalyst bed. Since the catalyst may be of large size withoutfearing attrition, a significant amount of catalyst may be used, forexample for in excess of the amount which could be retained in a slurryof particles in a conventional reactor. As a result, a greaterproportion of reactants can be reacted, and fewer unreacted startingmaterials may exit the catalyst bed.

The product take off is generally from the end of the reactor proximatethe exit from the catalyst bed. In vertical reactors with upward draughttube flow, the take off will preferably be below the bottom annularfilter element. Other product outlets are of course possible. Theproduct take off may be proportioned to require recirculation of aportion of the liquid exiting the filter bed back through the draughttube and from there again through the filter bed. In this manner, theconversion efficiency may be adjusted optimally.

The product stream may be routed directly topurification/recovery/recycle, or may serve as an inlet stream to afurther reactor to further maximize reaction. Where a second reactor isused, it may be of the same type or a different type. It may, forexample, be a simple tubular reactor with solid catalyst located betweenfilter elements. One or more additional inlet streams may be fed to thesecond reactor to minimize reactants present in the first reactorproduct stream, or to adjust the content of permanent gases tonon-explosive limits. For example, when the product stream containsunreacted hydrogen and oxygen, additional propylene may be added tolower the oxygen content. Methane, ethane or propane may be added toadjust limits of flammability, etc.

The product stream generally includes unreacted condensable or permanentgases, i.e. hydrogen, oxygen, propylene, nitrogen, methane, etc.,reactor solvent, and a product mixture. The product mixture containspropylene oxide and “propylene oxide equivalents,” i.e. propyleneglycol, dipropylene glycol, propylene glycol methyl ether, among others.The permanent gases may be removed by flashing or other techniques, andrecompressed and reused. Alternatively, they may be injected into aboiler for their fuel value. Propylene and condensable gases arepreferably recovered and recycled. Solvent, i.e. methanol, is alsopreferably recycled. Separation of propylene oxide from propylene oxideequivalents and other impurities may be performed using conventionaldistillation techniques.

The pore size of the filter elements may be varied to suit theparticular catalyst size and expected generation of fines by attrition.Suitable nominal pore sizes range from 0.1 μm to 40 μm, more preferablyfrom 1 μm to 20 μm, and most preferably in the range of 4 μm to 15 μm. Afeature of the present invention is that the catalyst size is of lesserimportance then in other systems where a catalyst slurry is circulated.Small catalyst particle sizes may be used with beds of increasedcross-sectional area to minimize pressure drop, for example. When suchsmall catalysts are used, filter element pore size should be on the lowside of the above range. When large size catalysts, i.e. from 2 μm to 40μm or more are used, pore size may be increased upwards. In catalysts ofsizes in the range of 6-12 μm, for example single large titaniumsilicalite crystals, a nominal pore size of 3-4 μm is believed adequate.Although it would appear that such filters would allow passage ofrelatively large “fines,” i.e. 2-3 μm, it has been found in practicethat such “coarse” filters do not generally allow passage of suchparticle sizes, perhaps due to the serpentine nature of the porescreated by the sintering process used to form the filter elements. Thepore size may be selected specifically to allow passage of some fines,particularly those of very small particle size. These fines willordinarily accumulate in the product distillation bottoms, and becauseof their small quantity, do not pose a separation problem.

By “substantially preventing” catalyst from leaving the catalyst bed ismeant that the majority of catalyst will be retained in the bed, as justdescribed above. Preferably, the pore size will be such to substantiallyprevent passage of catalyst particles having particles sizes in excessof 20% of the mean volumetric catalyst particle size charged to thereactor.

By the term “solid catalyst” as used herein is meant a particulatecatalyst which is located in the catalyst bed such that a flow of fluidmay be directed through the catalyst bed. The resistance to flow isrelated to the size of the catalyst particles and their geometry, theback pressure of the catalyst bed increasing per incrementalcross-sectional area as the particle size decreases. Catalyst particleswhich cannot pack closely together or which provide roughly sphericalgeometries allow increased fluid flow. For a given fluid flow in L/h,the flow may be increased or decreased with a given size and morphologyof catalyst particles by correspondingly increasing or decreasing thecross-sectional area of the annular space in which the catalystresidues. The flow will also be influenced by the length of the catalystbed. Since catalyst attrition is less severe in the reactors of thepresent invention, relatively large amounts of catalyst may be used,since the operational lifetime of the catalyst is dramaticallyincreased.

Preferred filter elements are made of sintered porous stainless steel.Such sintered filter products are available from Mott Corporation.

Numerous variations of the reactors disclosed are possible withoutdeparting from the spirit of the invention. For example, the reactorsmay be configured with heating and/or cooling elements, i.e. fins, plateexchangers, coils, loops, etc. Additional mixing elements may also beadded, as well as liquid recirculation loops, etc. What is requiredminimally is a draught tube reactor with an annular space between thedraught tube and the interior wall of the reactor, this annular spacehaving a solid catalyst disposed therein, and maintained in the annulusby means of at least one and preferably two filter elements, at leastone filter element located beyond the catalyst bed relative to thedirection of fluid flow. It should be noted that the term “central” usedin describing the location of the draft tube does not imply that thedraft tube is absolutely centered in the reactor. It may be offset fromcenter, for example. By “in the annular space” referring to filterelement location is meant either within or atop the annular space suchthat fluid is substantially prevented from entering or leaving theannular space, as the case may be, without passing through the filterelement. The filter element may, for example, be located above theannular space as shown in FIG. 3.

In certain preferred embodiments, flow of soluble reactants, solvents,and reaction products may be desired to occur across or through thewalls of the draught tube. Such embodiments may be particularly usefulwhere a catalyst basket is used which has porous sides, i.e. sides ofmetal mesh material. Such flow through the draught tube walls may befacilitated by perforating the walls with holes, slots, etc., or byconstructing all or a portion of the draught tube of porous material,i.e. of porous sintered stainless steel. These embodiments areillustrative and not limiting.

While embodiments of the invention have been illustrated and described,it is not intended that these embodiments illustrate and describe allpossible forms of the invention. Rather, the words used in thespecification are words of description rather than limitation, and it isunderstood that various changes may be made without departing from thespirit and scope of the invention. In the claims, the terms “a” and “an”mean “one or more than one” unless indicated otherwise.

1-14. (canceled)
 15. In a process for the epoxidation of an olefin inthe presence of a solid catalyst in a liquid reaction medium, theimprovement comprising epoxidizing said olefin in a reactor having acentral draft tube located within said reactor, at least one impellerlocated within said draft tube, an annular space between the draft tubeand a wall of the reactor, and a fluid flow path in only one directionwithin the draft tube and only countercurrent flow, relative to thedirection of fluid flow within the draft tube, through the annularspace, said reactor having a catalyst bed for the solid catalyst locatedwithin said annular space; b) a filter element in said annular spacepositioned downstream from said catalyst bed relative to the directionof fluid flow through said annular space, said filter element havingpores therein to allow fluid flow through said filter element, saidpores of sufficiently small size so to substantially prevent catalystparticles from leaving said catalyst bed; c) a second filter elementhaving pores therein to allow fluid flow through said second filterelement in said annular space upstream from said catalyst bed relativeto the direction of fluid flow through said annular space; said reactorhaving at least one inlet for an inlet feed stream and at least oneproduct outlet for a product stream, wherein the catalyst bed iscontained with a catalyst basket located within said reactor, saidcatalyst basket comprising: i) a central tube and a largerannulus-defining tube; and ii) a top ring and a bottom ring, each ofsaid top and bottom rings fixed to both said central tube and saidannulus-defining tube, the space between said central tube and saidannulus defining tube defining an annular space suitable for receiving acharge of said solid catalyst; wherein said filter element is positionedabove said top ring or within said top ring such that fluid enteringsaid annular space passes through said first filter element, said secondfilter element is positioned below said bottom ring or within saidbottom ring such that fluid leaving said annular space passes throughsaid second filter element, and said central tube surrounding said drafttube or comprising said draft tube, and separating olefin oxide from theproduct stream thereof.
 16. The process of claim 15, wherein said liquidreaction medium comprises an oxygen-containing solvent.
 17. The processof claim 15, wherein said solid catalyst comprises a titanium silicalitecatalyst.
 18. The process of claim 17, wherein said inlet feed streamcomprises hydrogen and oxygen and said solid catalyst comprises titaniumsilicalite treated to contain a noble metal.
 19. The process of claim17, wherein said solid catalyst is an agglomerate of the titaniumsilicalite bound together with a binder.
 20. The process of claim 15,wherein said draft tube contains one or more baffles oriented verticallywithin said draft tube, and proximate one or more of the impellers. 21.The process of claim 15, wherein one or more of the inlets terminatesproximate to an upstream end of said draft tube, relative to thedirection of fluid flow within said draft tube, or terminates withinsaid draft tube.
 22. The process of claim 15, wherein said productoutlet is located downstream from said annular space relative to thedirection of fluid flow through said annular space.
 23. The process ofclaim 15, wherein the reactor has a filter support plate positionedintermediate said top ring and said first filter element.
 24. Theprocess of claim 15, wherein the reactor has a filter support platepositioned intermediate said bottom ring and said second filter element.